What are some online courses for deep reinforcement learning?
This answer will point the reader to potentially useful resources, but I can't ensure that the courses are good (because I have never followed them).
Free

Reinforcement Learning in the Open AI Gym (a small course that you can find in the YouTube channel suggested in the other answer) by Phil Tabor

The free course Advanced Deep Learning &amp; Reinforcement Learning by DeepMind can be useful. See video 14, which discusses deep RL topics (e.q. DQN).

The free course CS234: Reinforcement Learning Winter 2020 can also be useful. In particular, lesson 6 is about deep reinforcement learning. Here's the course schedule.


Paid

Advanced AI: Deep Reinforcement Learning in Python (at Udemy) seems to be completely dedicated to DRL.

Machine Learning: Beginner Reinforcement Learning in Python  (at Udemy)

Deep Reinforcement Learning 2.0 (at Udemy)

Modern Reinforcement Learning: Deep Q Learning in PyTorch by Phil Tabor

Modern Reinforcement Learning: Actor-Critic Methods by Phil Tabor


In any case, if you are familiar with RL and deep learning topics, I encourage you to directly read the DQN papers (both by DeepMind folks)

Playing Atari with Deep Reinforcement Learning (2013)
Human-level control through deep reinforcement learning (2015)

Of course, deep RL isn't just DQN, but these are two very important papers that you should read.
You should also note that deep RL isn't anything special. It's just RL concepts combined with function approximators and deep learning tricks or deep neural networks. So, if you are really familiar with deep learning and RL (including the usage of neural networks to approximate policies and value functions), you don't need any course to understand deep RL concepts. You can just pick any deep RL paper and you can potentially understand it, although you may require more than 1-2 iterations (but that depends on the person).